In an MRI device, an examination object, usually a patient, is exposed to a uniform main magnetic field (B0 field) so that the magnetic moments of the nuclei within the examination object tend to rotate around the axis of the applied B0 field. Then, by transmitting an RF excitation pulse (B1 field) which is orthogonal to the B0 field, generated by means of an RF transmit antenna, the spins of the nuclei are excited and brought into phase, and a deflection of their net magnetization from the direction of the B0 field is obtained, so that a transversal component in relation to the longitudinal component of the net magnetization is generated.
After termination of the RF excitation pulse, the relaxation processes of the longitudinal and transversal components of the net magnetization begin. MR relaxation signals which are emitted by the transversal relaxation process, are detected by means of an MR/RF receive antenna. The received MR relaxation signals are then Fourier transformed to frequency-based MR spectrum signals and processed for generating an MR image of the nuclei of interest within an examination object.
In order to obtain a spatial selection of a slice or volume within the examination object and a spatial encoding of the received MR relaxation signals emanating from a slice or volume of interest, gradient magnetic fields are superimposed on the B0 field, having the same direction as the B0 field, but having gradients in the orthogonal x-, y- and z-directions.
For generating the gradient magnetic fields, a gradient magnet system comprising a number of gradient magnets in the form of gradient coils is provided which is typically operated by means of a gradient amplifier system for generating electrical currents for supplying the gradient coils. Usually, such gradient coil currents have a certain waveform which has to be produced by the gradient amplifier system very precisely. These current pulses have to be accurately controlled with a deviation of only several mA or less in order to ensure generation of the MRI images at a high quality and high spatial resolution and precision.
WO 2012/085777 A1 discloses a gradient amplifier system 21′ for accurately controlling the current in the gradient coil. As can be seen in FIG. 1, for each gradient coil of the gradient coils 203′, 204′, 205′ in x-, y- and z-directions, the gradient amplifier system 21′ comprises a gradient amplifier 11′ for driving it and a state-space feedback controller 10 operating in the digital domain for accurately controlling the current in it. The gradient amplifier 11′ has a gradient filter 111′ with a filter voltage and a filter current. For each gradient coil of the gradient coils 203′, 204′, 205′ in x-, y- and z-directions, the gradient amplifier system 21′ has three independent states to be measured, i.e. the current I′gc in that gradient coil, the filter voltage U′c and the filter current I′c. As in FIG. 1, all the three independent states are fed back to the state-space feedback controller 10′. By configuring the state-space feedback controller 10′, i.e. calculating proper control parameters of the state-space feedback controller 10′, the current in that gradient coil is accurately controlled.
An integrated tracking control system is found in “Digital controlled MOSFET gradient amplifier for magnetic resonance imaging” by SIQI LI ET AL., Electrical machines nad systems, 20 Aug. 2011. The integrated tracking control system consists of an an optimal feedback loop, a feed-forward controller and a nonlinear PI regulator. The PI regulator is implemented instead of the full-state feedback during the steady-state, so only the error signal between the reference current and the output gradient coil current is needed to be sampled precisely.